Method of carrier allocation to a plurality of cells in a cellular communication system

ABSTRACT

In a carrier allocation scheme for a cellular communications system, cells are divided into groups of different types (CG 1 , CG 2 , CG 3 , CG 4 ) and carriers (f xyz ) are divided into sets (f 1yz , f 2yz , f 3yz , f 4yz ) allocated respectively to the different types. Carriers may be re-used between different groups of the same type, and optionally within a group, subject to a minimum re-use distance rule. The allocation patterns may be varied independently between different groups of the same type. The scheme may be adapted to the demand for carriers over a predetermined period. Different allocation schemes applied to different carriers may be overlaid on the same cells. Different allocation schemes may be applied to different cells, provided that the minimum re-use distance rule is obeyed between schemes. The scheme may be applied to spot beams of a satellite communications system, or cells of a terrestrial cellular system.

TECHNICAL FIELD

The present invention relates to a method and apparatus for carrierallocation, involving re-use of carriers with spatial separation, suchas a re-use scheme for a multi-beam satellite or terrestrial cellularcommunications system.

BACKGROUND TO THE INVENTION

A conventional terrestrial cellular frequency re-use scheme isillustrated in FIGS. 1 and 2. The available frequency spectrum ispartitioned into frequency blocks f1 to f7 as shown in FIG. 2, each ofwhich is assigned to a corresponding cell in a repeating cell cluster,as shown in FIG. 1. In this specific example, a hexagonal 7-cell clusteris used. The distance d in FIG. 1 represents the re-use distance of thecluster, in other words the centre-to-centre distance between cells towhich the same frequency can be assigned. In this case, d=√{square rootover (7)}D where D is separation between centres of adjacent cells. Inany cellular re-use system, the minimum re-use distance is chosen sothat the interference between channels assigned the same frequency indifferent cells is kept below an acceptable threshold.

The re-use factor of a particular cellular re-use scheme can be definedas the number of cells in which a particular carrier can be used,divided by the total number of cells. In the conventional schemedescribed above, the re-use factor is 1/7. Other schemes may use 3, 4,19 or some other number of cells per cluster, depending on the desiredre-use distance. The conventional scheme is designed to maximize thisre-use factor.

The conventional re-use scheme described above does not provideefficient use of carriers where the demand for carriers is uneven acrossthe cell pattern. For example, there may be a demand for more carriersin one cell than in other cells within a cluster. However, it is notpossible to allocate more carriers to that one cell, while satisfyingthe minimum re-use distance requirement, without making the sameallocation of carriers to the corresponding cell in all the otherclusters.

In cellular communications systems, traffic demand distributions varyaccording to various temporal and spatial factors such as time of day,distribution of terminals, type of communication, and one-off events.For example, in a beam pattern which extends across different countriesand time zones, as is common with satellite beam patterns for example,peak traffic demand will occur at different times and different levelsin different beams.

These problems can be overcome to some extent by dynamic carrierassignment algorithms, for example temporary ‘borrowing’ of carriers forone cell experiencing a high demand from an adjacent cell having a lowdemand, without violating the minimum re-use distance requirement.However, borrowing is spectrally inefficient and it is usually notpossible or desirable to coordinate the borrowing of carriers acrossmultiple cells. Alternatively or additionally, asymmetric reuse schemescan be devised.

The document U.S. Pat. No. 6,269,245 discloses a carrier allocationscheme for a satellite cellular system, in which a 7-cell clusterpattern is imposed on the beams. Each cell is assigned a demand valuerepresenting the demand for carriers in each cell. For each cell inturn, a 19-cell re-use zone is centred on that cell and the maximumdemand values within the re-use zone are reduced until the total demandwithin the re-use zone is no greater than the total capacity of thesatellite system. Next, a static allocation scheme is constructed bydividing the available channels into a ‘preferred channel’ setcomprising three pools: a base demand pool for satisfying the minimumchannel requirements of the cells, a maximum demand pool for satisfyingthe maximum channel requirements of the cells, and a community pool forsatisfying extraordinary demand during anomalous events. The first andsecond pools are each divided into seven subpools for allocation to thecorresponding cells within each 7-cell cluster. The community pool isnot divided according to cell type. Channels are allocated in order ofpreference from the base demand pool, the maximum demand pool and thecommunity pool. A dynamic allocation scheme is applied where the staticscheme is unable to satisfy a demand in a specific cell. However, thefirst and second pools cannot be assigned flexibly to accommodateasymmetry in demand, while the community pool cannot be assigned withspectral efficiency because it is not divided according to cell type.

STATEMENT OF THE INVENTION

According to one aspect of the invention, there is provided a clusterre-use scheme which defines cell clusters of different types such thatclusters of the same type are separated by at least a minimum re-usedistance, wherein carriers are divided into a plurality of sets forallocation to the respective different cluster types. The allocation ofcarriers to cells within a cluster may be varied independently of theallocation within another cluster of the same type.

According to a second aspect of the present invention, there is provideda cluster group re-use scheme which defines cluster groups of differenttypes such that cluster groups of the same type are separated by atleast a minimum re-use distance, wherein carriers are divided into arespective plurality of sets for allocation to the respective clustergroup types. Each cluster group comprises a plurality of cell clusters,and carriers may be re-used between cell clusters of the same clustergroup according to a cell re-use scheme, which may vary betweendifferent cluster groups of the same type.

The first and second aspects of the invention may be applied todifferent subsets of a set of cells.

The first and second aspects of the invention may be applied todifferent sets of carriers in overlapping sets of cells.

The first and/or second aspects of the invention may be combined with acell re-use scheme applied to further carriers. The cell re-use schememay have a different clustering pattern from that of the first and/orsecond aspects, or the same clustering pattern as that of the firstand/or second aspects.

In either of the first and second aspects, the re-use scheme may bevaried, for example in response to actual or predicted demand.

According to a third aspect of the present invention, different re-useschemes such as cell, cluster and cluster group re-use schemes areoverlaid on the same set of cells, and different carrier pools areallocated to each re-use scheme.

The available carriers may be divided into pools allocated to thedifferent re-use schemes according to predicted demand. Carriers may beallocated to the cellular re-use pool so as to satisfy a component ofthe demand which is spatially constant over the cells. Carriers may beallocated to the cluster re-use pool so as to satisfy the demand inexcess of the spatially constant component. Carriers may be allocatedunder a cluster group re-use scheme so as to satisfy peak demand.

A borrowing scheme may also be devised so as to determine the prioritywith which carriers may be borrowed between pools, and optionallybetween cells, clusters and cluster groups.

The scope of the present invention extends to methods, apparatus,systems, signals, data structures and programs for any of the following,where applicable:

determining the re-use scheme;

varying the re-use scheme;

assigning carriers according to the re-use scheme; and

performing communications using carriers allocated according to there-use scheme.

The steps may be performed by any or any combination of one or moresatellites, earth stations, network coordination stations, networkoperation centres, terrestrial cellular base stations, switching centresor separate facilities connected or connectable to any of these.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 shows an example of a cellular re-use pattern;

FIG. 2 shows the frequency band partitioning of the re-use pattern;

FIG. 3 shows a cluster re-use pattern in a first embodiment of thepresent invention;

FIG. 4 shows a carrier allocation pattern in the first embodiment;

FIG. 5 shows the partitioning of carrier subsets in the firstembodiment;

FIG. 6 shows different partitioning of carrier subsets between differentclusters of the same type in the first embodiment;

FIG. 7 shows a cluster group re-use pattern in a second embodiment ofthe present invention;

FIG. 8 shows the partitioning of carrier subsets in the secondembodiment;

FIG. 9 illustrates a combined re-use scheme in which a cellular re-usescheme, a cluster re-use scheme according to the first embodiment and acluster group re-use scheme according to the second embodiment areoverlaid on the same set of cells and different pools of carriers areallocated to each;

FIG. 10 illustrates a combined re-use scheme in which a cellular re-usescheme, a cluster re-use scheme and a cluster group re-use scheme areused with different cells of the same cell pattern;

FIG. 11 is a flowchart of a generic allocation method which can beapplied to embodiments of the present invention;

FIG. 12 is a diagram of a cell pattern with cells references by clusterand cell number;

FIG. 13 is a chart illustrating a predicted demand for a selection ofcells, and levels used to determine the allocation of carriers underdifferent re-use schemes;

FIG. 14 is a flowchart showing an assignment process using an allocationplan in an embodiment of the invention;

FIG. 15 shows a geostationary satellite system within which embodimentsof the invention may be implemented;

FIG. 16 is a diagram of the architecture of a frequency planning part ofthe geostationary satellite system;

FIG. 17 shows a cluster re-use pattern according to the first embodimentapplied to the beam pattern of the geostationary satellite system;

FIG. 18 shows a first example of a cluster group re-use patternaccording to the second embodiment applied to the beam pattern of thegeostationary satellite system; and

FIG. 19 shows a second example of a cluster group re-use patternaccording to the second embodiment applied to the beam pattern of thegeostationary satellite system.

Cluster Re-Use Scheme

The first embodiment will now be described with reference to FIGS. 3 to5. In this embodiment, each cell is assigned to one cluster, and eachcluster is assigned to one of a plurality of cluster types C1, C2 andC3. As shown in FIG. 3, cluster type C1 comprises one cluster andcluster types C2 and C3 each comprise three non-adjacent clusters.

The allocation of the available frequency spectrum to clusters and cellswithin clusters is illustrated in FIGS. 4 to 6. The available frequencyspectrum f_(xy) is subdivided into three frequency bands f1 y, f2 y andf3 y, each of which is assigned to a corresponding cluster type C1, C2and C3. Each frequency band may be divided into seven frequency blockswhich are allocated respectively to the seven cells within a cluster, orthe frequencies may be assigned dynamically within the cluster accordingto demand.

As shown in FIG. 5, frequency band f_(1y) may be divided into frequencyblocks f₁₁, f₁₂, f₁₃, f₁₄, f₁₅, f₁₆ and f₁₇, frequency band f_(2y) isdivided into frequency blocks f₂₁, f₂₂, f₂₃, f₂₄, f₂₅, f₂₆ and f₂₇, andfrequency block f_(3y) is divided into frequency blocks f₃₁, f₃₂, f₃₃,f₃₄, f₃₅, f₃₆ and f₃₇. However, the same partition of frequency blocksdoes not need to be applied to all of the clusters of a cluster type,because these clusters are separated by at least one intervening clusterthat belongs to a different cluster type and therefore has a differentfrequency band assigned to it.

As shown in FIG. 4, the minimum re-use distance d between two clustersin the same group is the same as the minimum re-use distance betweenclusters in the prior art example shown in FIG. 2. Hence, thepartitioning of each cluster of a cluster type may be determinedindependently of the partitioning of any other cluster of the samecluster type.

For example, FIG. 6 shows three different partition schemes of thefrequency band f_(2y) applied respectively to three clusters within thecluster group C2, as shown in FIG. 4. In the first partition scheme, thefrequency band f_(2y) is divided into frequency blocks f₂₁, f₂₂, f₂₃,f₂₄, f₂₅, f₂₆ and f₂₇. In the second partition scheme, the frequencyband f_(2y) is divided into frequency blocks f₂₁′, f₂₂′, f₂₃′, f₂₄′,f₂₅′, f₂₆′ and f₂₇′. In the third partition scheme, the frequency bandf_(2y) is divided into frequency blocks f₂₁″, f₂₂″, f₂₃″, f₂₄″, f₂₅″,f₂₆″ and f₂₇″. Likewise, three different partition schemes may beapplied to the frequency band f3 y allocated to cluster group C3, asshown in FIG. 4. In the first partition scheme, the frequency bandf_(3x) is divided into frequency blocks f₃₁, f₃₂, f₃₃, f₃₄, f₃₅, f₃₆ andf₃₇. In the second partition scheme, the frequency band f_(3y) isdivided into frequency blocks f₃₁′, f₃₂′, f₃₃′, f₃₄′, f₃₅′, f₃₆′ andf₃₇′. In the third partition scheme, the frequency band f_(3y) isdivided into frequency blocks f₃₁″, f₃₂″, f₃₃″, f₃₄″, f₃₅″, f₃₆″ andf₃₇″.

The re-use factor of the first embodiment is 1/21, which is lower thanthat of the conventional cellular re-use arrangement, but allows greaterflexibility in the partition of a frequency band within each cluster andis therefore more suited to asymmetric allocation of frequencies withina cell pattern.

The frequencies allocated to each cluster may be allocated within thecluster according to one of the following schemes:

-   -   1) A fixed scheme in which the allocation of frequencies to the        cluster is divided statically among the cells.    -   2) A shared scheme in which the allocation of frequencies to the        cluster can be allocated to the individual cells within the        cluster dynamically according to demand within the cells. The        shared scheme may be fully dynamic where the allocation to        individual cells is determined entirely in response to actual        demand, or partially dynamic in which a predetermined priority        ranking is applied to the allocation of the frequencies to        individual cells, and a frequency is selected for assignment to        a communication within a cell based on the priority rankings of        the frequencies for that cell.    -   3) A hybrid scheme in which some of the frequencies allocated to        the cluster are allocated to the cells under the fixed scheme        and others of the frequencies are allocated under the shared        scheme.

It is not essential that the clusters of the first embodiment are allthe same shape and size, and contain the same number of cells, providedthat the minimum re-use distance d is applied to all frequencies.

Cluster Group Re-Use Scheme

A second embodiment will now be described with reference to FIGS. 7 and8. In this embodiment, the available spectrum f_(xyz) is subdivided intofour bands f_(1yz), f_(2yz), f_(3yz), f_(4yz), each of which isallocated to a respective cluster group type CG1, CG2, CG3, CG4,arranged as shown in FIG. 7. Each cluster group comprises three cellclusters, to each of which is allocated a respective sub-band—forexample, band f_(1yz) is divided into subbands f_(11z), f_(12z), andf_(13z). Within each cluster, the allocated subband may be divided intoseven blocks which are allocated respectively to the individual cells,or the frequencies may be assigned dynamically within the clusteraccording to demand. For example, subband f_(11z) is divided intofrequency blocks f₁₁₁, f₁₁₂, f₁₁₃, f₁₁₄, f₁₁₅, f₁₁₆, and f117. The sameband is allocated to each cluster group of the same cluster group type,but the partition of each band into subbands may be varied for eachcluster group independently of the other cluster groups of the sametype. Hence, the re-use factor is 1/84. The re-use factor can beincreased to 3/84 in this example, by re-using frequencies within eachcluster group using a cell re-use scheme.

It is not essential that the cluster groups of the second embodimentshould all be the same size and shape and have the same number of cells,provided that the minimum re-use distance d is applied to allfrequencies.

Furthermore, it is not essential that a set of frequencies that arecontinuous in frequency must be allocated to a particular cluster group,cluster or cell, subject to the physical limitations of the transceiversto which the frequencies are allocated.

The frequencies allocated to each cluster group may be allocated withinthe cluster group according to one of the following schemes:

-   -   1) A fixed scheme in which the allocation of frequencies to the        cluster group is divided statically among the clusters or cells        within the cluster group.    -   2) A shared scheme in which the allocation of frequencies to the        cluster group can be allocated to the individual cells or        clusters within the cluster group dynamically according to        demand. The shared scheme may be fully dynamic where the        allocation to individual cells or clusters is determined        entirely in response to actual demand, or partially dynamic in        which a predetermined priority ranking is applied to the        allocation of the frequencies to individual cells or clusters,        and a frequency is selected for assignment to a communication        within a cell based on the priority rankings of the frequencies        for that cell or the relevant cluster.    -   3) A hybrid scheme in which some of the frequencies allocated to        the cluster group are allocated under the fixed scheme and        others of the frequencies are allocated under the shared scheme.

Combined Re-Use Schemes

From the above description, it will be appreciated that better frequencyre-use is achieved with fewer group types and smaller numbers of cellsper group. On the other hand, more group types and more cells per groupallows for more asymmetry in frequency allocation across a cell pattern.For example, the conventional cell re-use scheme has one cell per groupand seven group types. The cluster re-use scheme has seven cells pergroup and three group types. The cluster group re-use scheme hastwenty-one cells per group and four group types.

A better match between frequency demand and allocation can be achievedby combining different re-use schemes having different re-use factorsand asymmetric capabilities. The asymmetry of allocations and thepermitted degree of sharing between the cells, clusters and clustergroups depends on the asymmetry of traffic demand. Resource dimensioningis performed primarily to match the traffic demand and secondarily toprovide a certain degree of flexibility and sharing according to thepredictability of the demand.

For example, as shown in FIG. 9, the available spectrum may be dividedinto three frequency pools: a cell pool P_(Ce) allocated under the cellre-use scheme, a cluster pool P_(Cl) allocated under the cluster re-usescheme, and a cluster group pool P_(CG) allocated under the clustergroup re-use scheme. For a given cell pattern, separate cell, clusterand cluster group re-use schemes are defined and the respective poolsare allocated under each scheme.

Alternatively or additionally, different re-use schemes may be appliedto different cells within a cell pattern; for example, in an area ofsymmetric demand between different clusters, a cell re-use pattern isapplied, while in areas of highly asymmetric demand, a cluster groupre-use pattern is applied, while observing the minimum re-use distancerule both within a re-use scheme and between re-use schemes. The numberof cells per cluster or cluster group may vary across the beam pattern,so long as the minimum reuse distance is observed for any givenfrequency.

For example, as shown in FIG. 10, part of a beam pattern is allocatedunder a cluster group re-use scheme with cluster groups CG1 to CG4,another part is allocated under a cluster reuse scheme with clusters C1to C3, and another part is allocated under a cell reuse scheme withcells 1-7. In FIG. 10, the same shading is used for some of the cells indifferent schemes. Cells with the same shading under different re-useschemes may have some frequencies allocated in common, although notnecessarily all. The shading illustrates how the minimum re-use distancemay be observed between neighbouring different allocation schemes.

Different combined neighbouring re-use schemes may be overlaid, usingdifferent pools of carriers.

A generic allocation method is shown in the flowchart of FIG. 11. Atstep S10, historical and other data are obtained. At step S20, thecarrier demands for each cell during the allocation period are predictedfrom the data obtained. At step S30, a combination of the cell, clusterand cluster group re-use schemes is found which matches the predictedcarrier demands per cell with the greatest efficiency, subject to anyadditional restrictions, such as regulatory restrictions of the use ofspecified frequencies in specified area and restrictions on the numberof carriers which can be used in each cell. At step S40, the availablespectrum is divided among the re-use schemes according to theirrequirements. At step S50, a borrowing scheme is determined so as toregulate how carriers are to be borrowed under a dynamic allocationscheme. At step S60, specific carriers are allocated to each cell,cluster and cluster group according to the re-use schemes and thespectrum dimensioning.

The process may be repeated for each successive allocation period. Thelength of the allocation periods may vary and may depend on the daily orweekly variation of demand. For example, different allocation schemesmay be defined for an off-peak period and a peak period of each day. Inany specific system, the flexibility of having frequently changingallocation schemes must be balanced with the inefficiency and complexityof the process involved in handing over between successive schemes.

Specific Allocation Method

A specific example of the allocation method will now be described withreference to FIGS. 12 and 13. In this example, the available carriersare divided into a cell re-use pool, a cluster re-use pool and a clustergroup re-use pool, as described above. As shown in FIG. 12, a cellpattern is divided into clusters, and each cell is identified by thereference Ccn, where:

c=cluster number

n=cell number within cluster

At step S10, historical data is accessed from records of the averagenumber of carriers in use for each cell and for each unit of time. Userdistribution data, showing the current number of users registered ineach cell, may be obtained from mobility management databases. Fixed orscheduled demand data, indicating capacity demands that have been leasedor reserved at specified times, may also be obtained.

At step S20, the predicted demand is calculated for each cell based onthe data obtained and the allocation period. For example, the historicalfluctuation of demand in each cell may be determined as a function oftime of day, day of the week, season, and the occurrence of a festivalor holiday period applicable to the coverage area of that cell, and thedemand may then be extrapolated for the current allocation period. Oneor more predictability factors may also be calculated for each cell,indicating the confidence level of the predicted demand. Hence, wherethe fluctuation of demand contains a large apparently random componentwhich is not a function of any of the parameters identified above, thepredictability factor is low. Conversely, where the fluctuation ofdemand is strongly dependent on one or more of these parameters, thepredictability factor is high. An example of predicted demand for aselection of the cells is shown in FIG. 13.

A weighting factor is calculated for each cell, for use in determiningthe allocation priority between different cells. The weighting factor iscalculated as a function of some or all of the following factors:

-   -   predicted load per cell    -   predictability factor of the load    -   density of users per cell

At step S30, a minimum traffic loading level L1 is determined, torepresent substantially the minimum cell loading level (in this caseL1=2). A corresponding number of carriers is allocated in each cell ofthe cell pattern under the cell re-use scheme: The level L1 representsthe substantially symmetrical component of the traffic loading. In oneexample, the minimum loading per cell is taken for each cluster, anaverage is taken for all the clusters, and the average is rounded up togive L1.

A static maximum traffic loading level L2 is then determined for eachcluster type, and the total predicted load between the levels L1 and L2is calculated for each cluster of the cluster type. For example, incluster 1 this total is 3+1+4+4+1=13. The maximum total predicted loadrequired by any of the clusters of that cluster type is then calculated,and a number of carriers sufficient to satisfy this maximum totalpredicted load is then allocated to each cluster of that cluster typeunder the cluster re-use scheme.

The cluster allocation may consist of a fixed allocation per cell, ashared allocation among the cells of the cluster or a hybrid allocationbetween the cells and the cluster. The degree of sharing is dependent onthe profile and predictability of the demand.

The peak traffic loading levels, above the level L2, are allocatedcarriers under the cluster group re-use scheme, and the boundariesbetween cluster groups are defined so as to maximize the carrier usageefficiency, taking into account the time of peak usage in each cell. Forexample, FIG. 13 shows that peak traffic loading levels exist in cellsC15 and C16. If these peaks occur substantially simultaneously, threecarriers will need to be allocated to satisfy this demand. However, ifthe peaks are not simultaneous, only two carriers need be allocated andmay be shared between the associated cells. This sharing may beprioritised according to a priority ranking for each cell. The priorityranking is determined by the predicted load level in each cell weightedby the weighting factor of that cell.

Other kinds of fixed, sharing or hybrid schemes as described above maybe used in this case.

If peak traffic loading levels occur in different cells separated by atleast the minimum re-use distance, these cells are preferably placedwithin the same cluster group or cluster group type so that carriers canbe re-used between the two peaks.

The levels L1 and L2 may be varied iteratively to match the predicteddemand as closely as possible while maximizing the re-use efficiency.

At step S40, the available frequency spectrum is partitioned between thecell, cluster and cluster group re-use schemes according to the numberof carriers required under each scheme for each cell, cluster andcluster group respectively, and subject to limitations imposed by thespecific system. For example, it may be advantageous in certain systemsto allocate a continuous block of spectrum containing a number offrequency channels to the same cell. This technique assists blockdown-conversion, and may be required in satellite systems where eachsatellite transponder, which transponds a continuous block of channels,can be assigned to only one beam at any one time.

At step S50, a borrowing scheme is devised to determine the borrowingpriority between the cell, cluster and cluster group pools. Thispriority may be based on the relative predicted excess capacity or‘cushion’ in each of the cell, cluster and cluster group pools. Forexample, a borrowing matrix may be defined as shown below in Table 1:

TABLE 1 Borrowing Matrix To Cluster From Cells Clusters Groups Cells 2 10 Clusters 1 0 2 Cluster Groups 2 1 2

Borrowing occurs preferably where indicated by priority 2, lesspreferably by priority 1 and not at all at priority 0. Each of thecategories of cells, clusters and cluster groups may be subdivided intoindividual cells, clusters and groups so as to give a priority forborrowing between any specific cell, cluster and cluster group.

If no channels can be borrowed under the borrowing scheme describedabove, any available channel can be borrowed subject to the minimumre-use distance rule.

Assignment of Carriers

The allocation plan is communicated to a network entity or entitieswhich receive requests for carriers for communication with terminals orother equipment such as gateways, assign specific carriers in responseto those requests, and update records of which carriers are currentlyassigned. The assigned carrier(s) may comprise one or more forwardcarriers and/or one or more return carriers, depending on the type ofservice requested. The functions performed by such network entities areshown in outline in the flowchart of FIG. 14.

At step S70, the network entity receives the allocation plan for therelevant period; if the network entity is responsible for assignment ofonly part of the allocation plan, only that part may be received. At thecommencement of the relevant period, the allocation plan is marked ascurrent. If required, a handover from the previous allocation plan isimplemented.

Steps S80 to S110 are repeated during the current allocation period andmay be run concurrently with other instances of these steps to handlemultiple concurrent requests.

At step S80 the network entity receives an assignment request. Therequest may be received from a terminal within the network, or may begenerated by a call or session initiated from another network to aterminal within the network. At step S90, the network entity determinesthe cell location of the terminal within the network. At step S100, thenetwork entity determines whether a carrier or carriers are availableand suitable for fulfilling the assignment request, either according tothe allocation plan or the borrowing scheme. If so, the carrier orcarriers are assigned at step S110 and the carriers are flagged asassigned at step S110. The assignment may involve sending an assignmentsignal indicating the assigned carrier or carriers to the terminalwithin the network. If no suitable carriers are available, a blockingcondition is signalled to the requesting party. In either case, a recordis made of the assignment or blocking for use in generating historicalusage records.

Although not shown in the flowchart of FIG. 14, the network entity mayalso receive a carrier release message from the terminal within thenetwork when the carrier is no longer required, or may detect that theassigned carriers are no longer in use. In either case, the carriers areflagged as available once more and a record made of the carrier releasefor use in generating the historical usage records.

Geostationary Satellite System

A specific application of the above embodiments to a geostationarysatellite system will now be described with reference to FIGS. 15 to 19.

FIG. 15 shows schematically a geostationary satellite communicationsystem including one or more satellite access stations (SAS) which actas gateways to other communications networks NET for communication withany of a large number of network terminals NT. Each SAS is able tocommunicate with a plurality of mobile terminals (MT) using radiofrequency (RF) channels retransmitted by one or more geostationarysatellites SAT.

The satellite SAT includes a beam forming network, receive antenna andtransmit antenna (not shown) which generate substantially congruentreceive and transmit beam patterns. Each beam pattern consists of aglobal beam GB, a small number of overlapping regional beams RB whichare narrower than, and fall substantially within, the global beam and alarge number of spot beams SB (only two of which are shown in FIG. 15,for clarity) which are narrower than the regional beams and may falleither within or outside the regional beams, but fall substantiallywithin the global beam. Each spot beam may or may not overlap anotherspot beam, and at least some of the spot beams are steerable so thattheir area of coverage on the earth's surface can be changed.

The satellite includes a transponder which maps each C-band frequencychannel received in a feeder link onto a corresponding L-band frequencychannel transmitted in a specified beam in a user link, and maps eachL-band frequency channel received in each beam in the user link onto acorresponding frequency channel in the feeder link. The mapping offrequency channels to beams can be varied under the control of a payloadcontrol station (PCS), which also controls the mapping of L-bandchannels in the user link to C-band channels in the feeder link. Thesatellite SAT acts as a ‘bent pipe’ and does not demodulate or modifythe format of the signals within each frequency channel.

The architecture of the frequency allocation system of the satellitesystem is shown in FIG. 16. A network operations centre (NOC) determinesthe current re-use scheme and the resultant frequency allocation plan.For each satellite SAT, a global resource manager (GRM) receives thefrequency allocation plan for that satellite The GRM controls the PCS toimplement the allocation scheme on the satellite, for example byconfiguring the satellite transponders to assign the specified frequencychannels to the allocated beams. For each SAS, a local resource manager(LRM) receives a subset of the frequency allocation plan representingfrequencies pre-allocated to that SAS and records the channelassignments made by the SAS, together with associated usage data.

The GRM receives the usage data from the LRMs. The usage data includestraffic reports over periods varying from one hour to three months, toreflect variations dependent on time of day, day of the week, holidayperiods and seasons, for example. The information may report, for eachbeam, on the quantity, type, duration, Quality of Service and/or datavolume of calls or sessions. Additionally, the reports may includestatistics on dropped calls or sessions, congestion or blockingincidents and overall service availability. The reports are sent to theNOC and are used as input for future frequency plans.

The NOC generates a new frequency plan hourly, daily or monthly, asneeded, and different schemes or parts of schemes of the frequency planmay be updated at different rates. For example, the cell re-use schememay be updated relatively infrequently, for example monthly, as thesymmetrical component of the carrier demand does not change rapidly. Thecluster re-use scheme may be updated more frequently, while the clustergroup re-use scheme may be updated frequently, for example hourly, toreflect short-term asymmetries across the cell pattern. Within aparticular scheme, the partition of the carriers within a group orbetween different groups may be varied, and/or the allocation of cellsto groups may be varied. Multiple frequency plans may be generated bythe NOC and distributed via the GRM, each tagged with a start time anddate indicating when that frequency plan is to be applied.

Each of the elements of the frequency allocation scheme may comprise oneor more computers running computer programs, together withcommunications interfaces to other elements to which they are connected;some of the elements may be combined within the same one or morecomputers.

An example of the cluster re-use scheme applied to the spot beam patternof the geostationary satellite system is shown in FIG. 17. The spot beampattern is designed to cover the terrestrial and coastal areas of thecoverage area of the satellite, and hence does not completely fill thecoverage area. The areas not covered by the spot beams may be servicedby the regional beams or the global beam.

A first example of the cluster group re-use scheme applied to the spotbeam pattern of the geostationary satellite system is shown in FIG. 18,while a second example of the cluster group re-use scheme applied to thespot beam pattern is shown in FIG. 19, in which the cluster group typeallocations of selected beams have been modified whilst keeping thecluster group allocation of others the same. It is apparent from FIGS.18 and 19 that there is great flexibility in the size and relativepositioning of the cluster group types, while still observing theminimum re-use distance rule.

Alternative Applications

Embodiments of the present invention may be applied to othergeostationary or geosynchronous satellite systems in which theallocation and assignment steps are carried out using a differentarchitecture, for example by processing means within the satellite.Embodiments may also be applied to non-geosynchronous satellite systems,for example earth-fixed satellite cellular systems in which thefrequency allocation is performed with reference to a notional cellularstructure defined with reference to the earth's surface rather than tothe satellite beams. Embodiments may also be applied to terrestrialcellular systems.

1. A method of allocating a plurality of carriers to a plurality ofcells in a wireless cellular communications system, the methodcomprising: dividing, at a network operations centre, the cells into aplurality of groups each comprising a contiguous plurality of saidcells, each group having one of a plurality of group types, such thatgroups of the same group type are separated by at least a predeterminedminimum re-use distance; dividing, at the network operations centre, thecarriers into a plurality of carrier sets corresponding respectively tothe plurality of group types; and allocating, at the network operationscentre, the carriers from each carrier set to the corresponding grouptype, such that at least some of the carriers are each allocated to morethan one of the groups of said plurality of groups, the groups of saidmore than one of the groups being of the same group type, none of saidcarriers is allocated to a plurality of cells mutually separated by lessthan said predetermined minimum re-use distance within the same group,and such that the allocation of said carriers among the cells of a firstof the groups of said plurality of groups differs from the allocation ofsaid carriers among the cells of a second of the groups of saidplurality of groups, the first and the second of the groups of saidplurality of groups being of the same group type.
 2. The method of claim1, wherein, for at least some of the groups of said plurality of groups,at least some of the carriers of the corresponding carrier set are eachallocated to a predetermined one of the cells of that group.
 3. Themethod of claim 1, wherein, for at least some of the groups of saidplurality of groups, at least some of the carriers of the correspondingcarrier set are allocated as available to any of the cells of thatgroup.
 4. The method of claim 1 wherein at least some of the carriersare allocated to a plurality of cells mutually separated by at leastsaid predetermined minimum re-use distance within the same group.
 5. Themethod of claim 1, wherein each of the groups of said plurality ofgroups has the same number and configuration of cells.
 6. The method ofclaim 5, wherein the number of said group types and carrier sets isthree.
 7. The method of claim 1, wherein at least one of the groupssurrounded by other groups comprises a different number of cells fromanother of said groups surrounded by other groups.
 8. The method ofclaim 7, wherein the number of said group types and carrier sets isfour.
 9. The method of claim 1, including varying the division of cellsinto groups as a function of time.
 10. The method of claim 1, includingvarying the division of carriers into carrier sets as a function oftime.
 11. The method of claim 1, including varying the allocation ofcarriers among cells within the same group as a function of time. 12.The method of claim 9, wherein the varying step is dependent on previoususage and/or availability of said carriers in said cells.
 13. A methodof allocating a first and a second plurality of carriers to a pluralityof cells, comprising: allocating, at a network operations centre, saidfirst plurality of carriers to the plurality of cells by a first method,wherein said first method comprises: dividing, at the network operationscentre, the cells into a plurality of first groups each comprising acontiguous plurality of said cells, each first group having the samenumber and configuration of cells and having one of a plurality of firstgroup types, such that first groups of the same type are separated by atleast a predetermined minimum re-use distance, dividing, at the networkoperations centre, said first plurality of carriers into a plurality offirst carrier sets corresponding respectively to the plurality of firstgroup types, and allocating, at the network operations centre, thecarriers from each first carrier set to the corresponding first grouptype such that at least some of the carriers of the first plurality ofcarriers are each allocated to a plurality of said first groups of thesame first group type, none of said carriers of the first plurality ofcarriers is allocated to a plurality of cells mutually separated by lessthan said minimum re-use distance within the same first group, and suchthat the allocation of said carriers of the first plurality of carriersamong the cells of one of said first groups differs from the allocationof said carriers of the first plurality of carriers among the cells ofanother one of said first groups of the same type; and allocating, atthe network operations centre, said second plurality of carriers to theplurality of cells by a second method, wherein said second methodcomprises: dividing, at the network operations centre, the cells into aplurality of second groups each comprising a contiguous plurality ofsaid cells, wherein at least one of said second groups surrounded byother second groups comprises a different number of cells from anotherof said second groups surrounded by other second groups, each secondgroup having one of a plurality of second group types, such that secondgroups of the same type are separated by at least a predeterminedminimum re-use distance, dividing, at the network operations centre,said second plurality of carriers into a plurality of second carriersets corresponding respectively to the plurality of second group types,and allocating, at the network operations centre, the carriers from eachsecond carrier set to the corresponding second group type such that atleast some of the carriers of the second plurality of carriers are eachallocated to a plurality of said second groups of the same second grouptype, none of said carriers of the second plurality of carriers isallocated to a plurality of cells mutually separated by less than saidminimum re-use distance within the same second group, and such that theallocation of said carriers of the second plurality of carriers amongthe cells of one of said second groups differs from the allocation ofsaid carriers of the second plurality of carriers among the cells ofanother one of said second groups of the same type.
 14. The method ofclaim 13, further comprising allocating a further plurality of carriersto said plurality of cells by dividing the cells into a plurality ofcell types such that cells of the same type are separated by at leastthe minimum re-use distance, and dividing the further plurality ofcarriers into a respective plurality of further carrier sets allocatedrespectively to said plurality of cell types.
 15. A method of allocatinga plurality of carriers to a first and a second plurality of cellswithin the same cellular communication system, comprising: allocating,at a network operations centre, said plurality of carriers to said firstplurality of cells by a first method and allocating said plurality ofcarriers to said second plurality of cells according to a second methodsuch that none of the carriers is allocated to one of said firstplurality of cells and to one of said second plurality of cellsseparated by less than said minimum re-use distance, wherein said firstmethod comprises: dividing, at the network operating centre, said firstplurality of cells into a plurality of first groups each comprising acontiguous plurality of said first plurality of cells, each first grouphaving the same number and configuration of cells and having one of aplurality of first group types, such that first groups of the same typeare separated by at least a predetermined minimum re-use distance,dividing, at the network operating centre, said carriers into aplurality of carrier sets corresponding respectively to the plurality offirst group types, and allocating, at the network operations centre, thecarriers from each carrier set to the corresponding group type such thatat least some of the carriers are each allocated to a plurality of saidfirst groups of the same first group type, none of said carriers isallocated to a plurality of cells mutually separated by less than saidminimum re-use distance within the same first group, and such that theallocation of said carriers among the cells of one of said first groupsdiffers from the allocation of said carriers among the cells of anotherone of said first groups of the same type; and wherein said secondmethod comprises: dividing, at the network operating centre, said secondplurality of cells into a plurality of second groups each comprising acontiguous plurality of said second plurality of cells, wherein at leastone of the second groups surrounded by other second groups comprises adifferent number of cells from another of said second groups surroundedby other second groups, each second group having one of a plurality ofsecond group types, such that second groups of the same type areseparated by at least a predetermined minimum re-use distance, dividing,at the network operating centre, said carriers into a plurality ofcarrier sets corresponding respectively to the plurality of second grouptypes, and allocating, at the network operations centre, said carriersfrom each carrier set to the corresponding group type such that at leastsome of said carriers are each allocated to a plurality of said secondgroups of the same second group type, none of said carriers is allocatedto a plurality of cells mutually separated by less than said minimumre-use distance within the same second group, and such that theallocation of said carriers among the cells of one of said second groupsdiffers from the allocation of said carriers among the cells of anotherone of said second groups of the same type.
 16. A method of allocating aplurality of carriers to a plurality of cells in a wireless cellularcommunications system, the method comprising: determining, at a networkoperating centre, a first re-use scheme in which the cells are dividedinto a plurality of first groups, each first group having one of aplurality of first group types, such that first groups of the same typeare separated by at least a predetermined minimum reuse distance;determining, at the network operating centre, a second re-use scheme inwhich the cells are divided into a plurality of second groups, eachsecond group having one of a plurality of second group types, such thatsecond groups of the same type are separated by at least said minimumre-use distance, wherein the first groups are not coterminous with thesecond groups; dividing, at the network operating centre, the pluralityof carriers into at least first and second carrier pools; andallocating, at the network operating centre, the first and second poolsrespectively under the first and second re-use schemes such that thefirst pool is divided into a plurality of first carrier sets allocatedrespectively to the plurality of first group types and the second poolis divided into a plurality of second carrier sets allocatedrespectively to the plurality of second group types.
 17. The method ofclaim 16, wherein the first groups each comprise a mutually differentone of said cells.
 18. The method of claim 16, wherein the second groupseach comprise a mutually different plurality of said cells.
 19. Themethod of claim 16 wherein the carriers are divided between the firstand second carrier pools according to a predicted demand among saidcells.
 20. The method of claim 16, wherein the carriers are dividedbetween the first and second carrier pools so as to maximize the re-useefficiency of the carriers.
 21. A method of allocating a plurality ofcarriers to a plurality of cells in a wireless cellular communicationssystem, the method comprising: determining, at a network operatingcentre, a first re-use scheme in which the cells are divided into aplurality of cell types, such that cells of the same type are separatedby at least a predetermined minimum reuse distance; determining, at thenetwork operating centre, a second re-use scheme in which the cells aredivided into a plurality of groups each comprising a plurality ofcontiguous cells, each group having one of a plurality of group types,such that groups of the same type are separated by at least said minimumre-use distance; dividing, at the network operating centre, theplurality of carriers into at least first and second carrier pools; andallocating the first and second pools respectively under the first andsecond re-use schemes such that the first pool is divided into aplurality of first carrier sets allocated respectively to the pluralityof cell types and the second pool is divided into a plurality of secondcarrier sets allocated respectively to the plurality of group types. 22.The method of claim 21, wherein the allocation of the plurality ofcarriers is divided between the first and second carrier pools accordingto a predicted demand among said cells.
 23. The method of claim 21,wherein a constant level of said carriers is allocated to each of thecells under said first re-use scheme.
 24. The method of claim 23,wherein said constant level corresponds substantially to a constantcomponent of said predicted demand across the cells.
 25. The method ofclaim 21, further comprising, for each of said group types, determininga maximum static demand level, determining a proportion of saidpredicted demand above said constant level up to said maximum staticdemand level, and dividing said carriers to the second pool so as tosatisfy said proportion of the predicted demand.
 26. A method ofallocating a plurality of carriers to a plurality of cells in a wirelesscellular communications system, the method comprising: determining, at anetwork operating centre, a first re-use scheme in which the cells aredivided into a plurality of cell types, such that cells of the same typeare separated by at least a predetermined minimum re-use distance,wherein a constant level of said carriers is allocated to each of thecells under said first re-use scheme; determining, at the networkoperating centre, a second re-use scheme in which the cells are dividedinto a plurality of groups each comprising a plurality of contiguouscells, each group having one of a plurality of group types, such thatgroups of the same type are separated by at least said minimum re-usedistance; dividing, at the network operating centre, the plurality ofcarriers into at least first and second carrier pools according to apredicted demand among the cells; and allocating, at the networkoperating centre, the first and second carrier pools respectively underthe first and second re-use schemes such that the first carrier pool isdivided into a plurality of first carrier sets allocated respectively tothe plurality of cell types and the second carrier pool is divided intoa plurality of second carrier sets allocated respectively to theplurality of group types; for each of said group types, determining, atthe network operating centre, a maximum static demand level, determininga proportion of said predicted demand above said constant level up tosaid maximum static demand level, and dividing said carriers to thesecond carrier pool so as to satisfy said proportion of the predicteddemand; and determining, at the network operating centre, a third re-usescheme in which the cells are divided into a plurality of further groupseach comprising a plurality of contiguous cells, each further grouphaving one of a plurality of further group types, such that third groupsof the same type are separated by at least said minimum re-use distance,wherein the further groups are not coterminous with the groups of thethird re-use scheme; allocating to a third carrier pool at least some ofthe carriers not allocated to the first and second carrier pools; andallocating the third carrier pool under the third re-use scheme bydividing the third carrier pool into a plurality of third carrier setsallocated respectively to said plurality of further group types.
 27. Themethod of claim 26, including determining a peak demand comprising saidpredicted demand above said maximum static demand level, wherein saidcarriers are allocated to said third carrier pool so as to satisfy saidpeak demand.
 28. A method of allocating a plurality of carriers to aplurality of cells in a wireless cellular communications system, themethod comprising: determining, at a network operating centre, a re-usescheme in which the cells are divided into a plurality of groups eachcomprising a plurality of contiguous cells, each group having one of aplurality of group types, such that groups of the same type areseparated by at least a minimum re-use distance; determining, at thenetwork operating centre, a further re-use scheme, in which the cellsare divided into a plurality of further groups each comprising aplurality of contiguous cells, each further group having one of aplurality of further group types, such that further groups of the sametype are separated by at least said minimum re-use distance, anddividing, at the network operating centre, some or all of the carriersbetween a carrier pool and a further carrier pool allocated respectivelyunder the re-use scheme and the further re-use scheme such that thefurther carrier pool satisfies a predicted peak demand among said cells.29. The method of claim 28, wherein the allocation of said furthercarrier pool under said further re-use scheme includes a priorityweighting between said cells.
 30. The method of claim 29, wherein saidpriority weighting is dependent on the relative size of said peak demandbetween the respective cells.
 31. The method of claim 29, wherein theallocation of said further carrier pool between said cells under saidfurther re-use scheme is dependent on the relative timing of saidpredicted peak demand between the respective cells.
 32. The method ofclaim 29, including determining borrowing priorities between saidcarrier pools.
 33. The method of claim 32, wherein said borrowingpriorities are dependent on the predicted excess capacity of said pools.34. The method of claim 32, wherein said borrowing priorities includeborrowing priorities between cells and/or groups of each said re-usescheme.
 35. A method of allocating a plurality of carriers to aplurality of cells in a wireless cellular communications system, themethod comprising: determining, at a network operating centre, a firstre-use scheme in which the cells are divided into a plurality of firsttypes, such that cells of the same first type are separated by at leasta predetermined minimum reuse distance; determining, at the networkoperating centre, a second re-use scheme in which the cells are dividedinto a plurality of second groups each comprising a plurality ofcontiguous cells, each second group having one of a plurality of secondgroup types, such that second groups of the same type are separated byat least said minimum re-use distance; determining, at the networkoperating centre, a third re-use scheme in which the cells are dividedinto a plurality of third groups each comprising a plurality ofcontiguous cells, each third group having one of a plurality of thirdgroup types, such that third groups of the same type are separated by atleast said minimum re-use distance, wherein said third groups are notcoterminous with the second groups; and dividing, at the networkoperating centre, some or all of the allocation of the plurality ofcarriers between the first, second and third re-use schemes.
 36. Amethod of allocating a plurality of carriers to a plurality of cells ina wireless cellular communications system, in which the allocation,determined by a network operating centre, of at least one of thecarriers is shared between two or more of said cells, separated by lessthan a predetermined reuse distance, according to a weighting factor ofeach of said two or more cells.
 37. The method of claim 36, wherein saidweighting factor is dependent on one or more of the predicted load, thepredictability of the load and the density of users in the correspondingcell.
 38. The method of claim 36, further including outputting carrierallocation data representing the allocation of the carriers to thecells.
 39. Carrier allocation data output by the method of claim 38embodied in a computer readable medium.
 40. A method of assigning aplurality of carriers to a plurality of wireless communication devicesin a plurality of cells of a wireless communications system, comprising:receiving, at a network operating centre, some or all of the carrierallocation data of claim 39, and assigning, at the network operatingcentre, the carriers to the wireless communication devices according tothe received carrier allocation data and the location of said wirelesscommunication devices within the cells.
 41. The method of claim 40,including transmitting assignment signals representing the assignment ofthe carriers to the wireless communication devices.